Multicellular organisms are dependent on advanced mechanisms of information transfer between cells and body compartments. The information that is transmitted can be highly complex and can result in the alteration of genetic programs involved in cellular differentiation, proliferation, or reproduction. The signals, or hormones, are often low molecular weight molecules, such as peptides, fatty acid, or cholesterol derivatives.
Many of these signals produce their effects by ultimately changing the transcription of specific genes. One well-studied group of proteins that mediate a cell's response to a variety of signals is the family of transcription factors known as nuclear receptors, hereinafter referred to often as “NR.” Members of this group include receptors for steroid hormones, vitamin D, ecdysone, cis and trans retinoic acid, thyroid hormone, bile acids, cholesterol-derivatives, fatty acids (and other peroxisomal proliferators), as well as so-called orphan receptors, proteins that are structurally similar to other members of this group, but for which no ligands are known. Orphan receptors may be indicative of unknown signalling pathways in the cell or may be nuclear receptors that function without ligand activation. The activation of transcription by some of these orphan receptors may occur in the absence of an exogenous ligand and/or through signal transduction pathways originating from the cell surface.
In general, three functional domains have been defined in NRs. An amino terminal domain is believed to have some regulatory function. It is followed by a DNA-binding domain (hereinafter referred to as “DBD”), which usually comprises two zinc finger elements and recognizes a specific Hormone Responsive Element (hereinafter referred to as “HRE”) within the promoters of responsive genes. Specific amino acid residues in the “DBD” have been shown to confer DNA sequence binding specificity. A ligand-binding-domain (hereinafter referred to as “LBD”) is at the carboxy-terminal region of known NRs.
In the absence of hormone, the LBD appears to interfere with the interaction of the DBD with its HRE. Hormone binding seems to result in a conformational change in the NR and thus opens this interference. A NR without the LBD constitutively activates transcription but at a low level.
Coactivators or transcriptional activators are proposed to bridge between sequence specific transcription factors, and the basal transcription machinery and in addition to influence the chromatin structure of a target cell. Several proteins like SRC-1, ACTR, and Grip1 interact with NRs in a ligand enhanced manner.
Nuclear receptor modulators like steroid hormones affect the growth and function of specific cells by binding to intracellular receptors and forming nuclear receptor-ligand complexes. Nuclear receptor-hormone complexes then interact with a HRE in the control region of specific genes and alter specific gene expression.
The Farnesoid X Receptor alpha (hereinafter also often referred to as NR1H4 when referring to the human receptor) is a prototypical type 2 nuclear receptor which activates genes upon binding to a promoter region of target genes in a heterodimeric fashion with Retinoid X Receptor. The relevant physiological ligands of NR1H4 are bile acids. The most potent one is chenodeoxycholic acid (CDCA), which regulates the expression of several genes that participate in bile acid homeostasis. Farnesol and derivatives, together called farnesoids, are originally described to activate the rat orthologue at high concentration but they do not activate the human or mouse receptor. FXR is expressed in the liver, throughout the entire gastrointestinal tract including the esophagus, stomach, duodenum, small intestine, colon, ovary, adrenal gland and kidney. Beyond controlling intracellular gene expression, FXR seems to be also involved in paracrine and endocrine signalling by upregulating the expression of the cytokine Fibroblast Growth Factor 15 (rodents) or 19 (monkeys, humans A).
Although numerous FXR agonists are known, there is a need for improved FXR agonists.